Securing information safety is an urgent task of the network society, and quantum cryptographic communication is expected as a core technology.
However, the current safety key generation rate is not sufficient and not practical. To realize a practical safety key generation rate, it is essential to achieve higher performance of a photon detector, which is an urgent task.
Conventionally, an InGaAs avalanche photodiode (APD) has been widely used for photon detection in the communication wavelength band (1,550 nm), but there are problems of afterpulse generation and a large dark count. These problems are factors in increase of an error rate of key generation, but it is difficult to avoid these problems due to an operation principle of the APD.
Conventionally, a superconducting strip photon detector (SSPD) has also been used for photon detection (see US Unexamined Patent Application Publication No. 2005/0051726 and Japanese Patent Application Laid-Open Publication No. 2011-176159). As features of the SSPD, there is no afterpulse generation, and a dark count is relatively small. Accordingly, the SSPD is a major photon detector which realizes the quantum cryptographic communication.
However, it is difficult for the SSPD to achieve both high sensitivity and a low dark count rate due to an operation principle thereof.
FIG. 1(a) is a diagram illustrating an equivalent circuit of a conventional SSPD photon detection device. The SSPD is a photon detection device having a superconducting stripline with a thickness of several nanometers and a width of several tens to hundreds of nanometers, and is operated by constantly flowing a bias current through the superconducting stripline. When a photon is made incident on the superconducting stripline through which the bias current flows, a complete normal conductive region is formed in a width direction of the superconducting stripline (see N in FIG. 1(a)). Thereupon, the bias current cannot flow in a forward direction, and a current pulse is output to a measurement system. In the SSPD, this current pulse is used as a photon detection signal.
In this case, probability (quantum efficiency) of generation of the current pulse when the photon is made incident on the superconducting stripline, that is, the sensitivity depends on a magnitude of the bias current as illustrated in FIG. 1(b) (see C. M. Natarajan et al., Supercond. Sci. Technol. 25, 063001 (2012)). The larger the bias current, the higher the quantum efficiency (see the left axis), but the dark count also increases at the same time (see the right axis). FIG. 1(b) is a diagram illustrating bias current dependency of quantum efficiency and bias current dependency of a dark count rate.
Thus, it is impossible to achieve both high sensitivity and a low dark count rate in the SSPD photon detection device.
Moreover, since the current pulse is used as a principle of photon detection, it is necessary to use a low noise amplifier for amplifying the current pulse illustrated in FIG. 1(a). The low noise amplifier, however, adversely affects jitter of a detection system.
Furthermore, a fall time T of the current pulse which is the principle of the photon detection is limited by a magnitude of an inductance L of the superconducting stripline. The inductance L is inversely proportional to a cross-sectional area of an entire length of the superconducting stripline. To reduce the magnitude of the inductance L, it is necessary to enlarge the cross-sectional area of the superconducting stripline. However, from the viewpoint of the sensitivity (quantum efficiency), the cross-sectional area of the superconducting stripline must be small. If the cross-sectional area is large, formation of the normal conductive region upon the incidence of the photon becomes incomplete, so that the sensitivity is lowered. For this reason, the inductance L must be increased. Accordingly, the fall of the current pulse is slow, and the fall time T becomes long, so that a response time constant of the system becomes several tens of nanoseconds which is considered to be large, whereby a counting rate of only substantially several hundreds of megahertz is obtained.